Herbaceous grafting is often applied to high-value vegetable crops such as watermelon (Citrullus lanatus), eggplant (Solanum melongena), and tomato worldwide for added vigor, stress tolerance, and disease resistance (Lee, 1994, 2003, 2007; Louws et al., 2010; Rivero et al., 2003). Although grafting may be very useful for tomato growers in the central region of the United States, there is limited availability of grafted plants propagated in the United States for commercial tomato fruit production. Results from a survey of fruit and vegetable growers at the 2014 Great Plains Growers Conference (St. Joseph, MO) showed that, of the 265 survey participants (65% of whom are growing in high tunnels), 19% are using grafted plants to some degree, but an additional 56% were interested in learning more on incorporating grafted plants into their production. Furthermore, 24% of those surveyed indicated that they were not using grafted plants, but would like to, indicating the potential impact that increased plant availability could have on these farmers.
Large-scale nurseries in Canada have been selling grafted tomato plants in the United States (Kubota et al., 2008). However, meeting the low volume and/or specialized nature of orders that are needed for small-acreage and/or farmer’s-market growers may be difficult due to constraints associated with specialized scion/rootstock cultivar selection, timing requirements of the grower (e.g., multiple small shipments vs. one large shipment), seasonal variation and resulting planting date, shipping during periods of inclement weather, and issues associated with seed sanitation certification programs. Although large-scale grafting nurseries will most likely play a role in the production of grafted plants (Kubota et al., 2008), there still exists a strong need to advance knowledge related to propagation of grafted plants for small-acreage growers who wish to graft their own plants (1000–15,000 plants annually). By grafting plants on the farm, growers can maintain control of their propagation systems (e.g., plant delivery date, need for specialty scion or rootstock cultivars, ease of providing plants for succession plantings, organic certification, etc.) and could potentially increase on-farm revenue when plants are sold to nearby farmers and/or gardeners. In the 2014 survey at the Great Plains Growers Conference mentioned above (n = 265), 47% of respondents indicated that they would prefer to grow their own grafted plants, whereas 25% indicated they would prefer to purchase grafted plants. An additional 28% were not interested in growing or purchasing grafted plants. These data highlight the potential impact that development of accessible propagation systems could have at overcoming plant availability, which is a significant barrier in the adoption of grafting in the United States.
The most popular grafting method for tomato is the tube-grafting technique (also known as splice grafting or Japanese top grafting) due to its efficiency and simplicity (Oda, 1995). This process (Bumgarner and Kleinhenz, 2014; Rivard and Louws, 2011) requires that the rootstock and scion (with 1.5- to 2-mm stem diameters) are cut at ≈60° to 75° angles (Bausher, 2013), held together with a silicon grafting clip, then placed in an environment (“healing chamber”) with high humidity and low light to promote a connection between the vascular tissues and prevent scion wilt (Oda, 2007).
Healing chamber management can be difficult. In particular, overheating of healing chambers inside of greenhouses has been problematic for grafted tomato transplant growers in the United States that are experimenting with grafting (C.L. Rivard, unpublished data). Not only do healing chambers require increased labor and management, but they also add to the overall cost of producing a grafted transplant as it requires additional materials and labor—accounting for 6.1% to 6.5% of the additional costs encountered when producing grafted tomato plants (Rivard et al., 2010). On the basis of industry and extension technical publications, the current recommended temperature range for healing chambers is 28 to 29 °C, and the recommended range for relative humidity is 85% to 100% (De Ruiter Seeds, 2006; Rivard and Louws, 2011). Johnson and Miles (2011) noted that tomato might be more tolerant of higher temperature and variable relative humidity (and thereby require less maintenance) than other horticultural crops such as watermelon. However, systematic experiments have not been published that quantify the effects of temperature and humidity on graft survival. Furthermore, the successful adoption of grafted propagation by small-scale tomato growers requires simple effective techniques that work within propagation facilities that have limited climate control and/or lighting available.
The function of the healing chamber is to reduce water stress on the scion tissue so that it can survive while the graft union fuses. In addition to modifications to the healing chamber, removal of leaf and/or tissue may prevent excessive evapotranspiration and therefore reduce or eliminate the need for microclimate management. The leaves probably play an important role in graft union formation due to their ability to provide photosynthate during the formation of the graft union. Leaf removal is recommended as a best management practice for the cleft and splice method (Bumgarner and Kleinhenz, 2014). Reducing water stress on the scion tissue by removing leaf area may reduce or eliminate reliance on the healing chamber, which could potentially facilitate more grafting success for a propagator with limited facilities.
In addition to reducing water stress, the removal of the entire scion meristem could expedite the process of growing a plant with two “leaders.” Two on-farm case studies from North Carolina and Pennsylvania were published by Rivard et al. (2010) to determine the estimated production costs for grafted tomato plants. The production model used by a commercial propagator in Pennsylvania included removal of the meristem 10 d postgrafting to encourage two “leaders” for vertical trellising. This practice is common for greenhouse production and high tunnel production of indeterminate cultivars with grafted plants (Besri, 2003; Kubota et al., 2008). By removing the meristem after the grafted plant has been healed, additional regrowth time is required, and this can set back planting in the field/greenhouse 10 to 14 d and/or slow early growth. The removal of the meristem during the grafting procedure could result in a grafted plant with two leaders, and would reduce the added time for regrowth posthealing. It would also potentially help reduce water loss by the scion, similarly to LR, which could facilitate less intensively managed healing chambers.
Clearly, there exists a strong need to determine not only environmental factors related to healing chambers, but also information related to LR and SR to help facilitate the development of successful propagation systems for small-acreage tomato growers. Therefore, the overall goals of this study were to 1) determine if healing chamber design (supplemental humidity and covering) affects graft survival, 2) describe how healing chamber design affects the healing chamber microclimate, and 3) investigate if scion shoot and/or LR affect graft survival in various healing chamber environments.
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